In machinery for forming metal and wood, some apparatus for reversing the rotational direction of either the workpiece or the tool, and for varying the rotational speed of the workpiece or tool as appropriate for the task to be accomplished. For example, threading operations require a much lower speed than turning or boring operations, and sanding or polishing operations require a yet higher speed. Previously, speed and direction control for such tasks on metalworking machinery was accomplished with a constant-speed unidirectional or reversible motor, together with repositionable drive belts or variable-diameter pulleys, sometimes in combination with interchangeable or shiftable gearing. Also, a separate rotary potentiometer or variable resistor for speed control and a reversing switch for direction control have been provided. As will be apparent, such an arrangement is undesirable, since it permits the unintentional starting of a rotating element at a high rate of speed, and also permits the reversal of rotation at a high rate of speed, resulting in high mechanical stresses and electrical stresses upon reversing switch contacts. In an attempt to prevent reversal of rotation at high speed, special electrical switches, known as hesitation switches, have been provided, which prevent continuous operation from a control switch from a first direction position to a second direction position. Such switches are relatively expensive, and are seldom used.
The instant invention provides a solution to these and other deficiencies of known motor control devices.